linux-kernel - Re: [PATCH RESEND] arm: topology: Fix missing clock-frequency property warning

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Date: Thu, 1 Feb 2024 16:03:59 +0100
From: Stefan Wiehler <stefan.wiehler@...ia.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
 Russell King <linux@...linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org
Subject: Re: [PATCH RESEND] arm: topology: Fix missing clock-frequency
 property warning

> Does this mean the /cpus property is like a default for when a CPU node
> doesn't specify the clock frequency, or does it mean that the /cpus
> property should only exist when all the values for each CPU are
> identical and thus the individual CPU node clock frequency should
> not be specified.

Good question, the device tree specification in Section 3.7 [1] says:

 > The /cpus node may contain properties that are common across cpu
nodes. See Section 3.8 for details.

And in Section 3.8 [2]:

 > Properties that have identical values across cpu nodes may be placed
 > in the /cpus node instead. A client program must first examine a
 > specific cpu node, but if an expected property is not found then it
 > should look at the parent /cpus node. This results in a less verbose
 > representation of properties which are identical across all CPUs.

So I think it is pretty clear that it should only be used for 
common/identical values.

> Aren't you adding new property? Is it already documented in the
> bindings? After a quick look I think this is not documented.

You are right, clock-frequency is not mentioned neither in arm/cpus.yaml 
nor in any other <arch>/cpus.yaml binding, but the DT spec has it as a 
required property [3]. Should I add clock-frequency to all 
<arch>/cpus.yaml bindings? Only the ARM one explicitly mentions 
following the DT spec.

Kind regards,

Stefan

[1] 
https://devicetree-specification.readthedocs.io/en/latest/chapter3-devicenodes.html#cpus-node-properties
[2] 
https://devicetree-specification.readthedocs.io/en/latest/chapter3-devicenodes.html#cpus-cpu-node-properties
[3] 
https://devicetree-specification.readthedocs.io/en/latest/chapter3-devicenodes.html#general-properties-of-cpus-cpu-nodes